Question

Which of the following 3d-metal ion will give the lowest enthalpy of hydration $\left({\Delta }_{\text{hyd}}\text{H}\right)$ when dissolved in water ?

• A. ${\text{Mn}}^{2+}$
• B. ${\text{Fe}}^{2+}$
• C. ${\text{Cr}}^{2+}$
• D. ${\text{Co}}^{2+}$

Answer 1

The correct option is A ${\text{Mn}}^{2+}$

Among the given ions, $M{n}^{2+}$ has the least charge to radius ratio due to which it gives lowest enthalpy of hydration on dissolving in water,

$\begin{array}{cc}\text{Ion}& \Delta {\text{H}}_{\text{Hyd.}}^{\text{o}}\left(\text{kJ}/\text{mol}\right)\\ {\text{Cr}}^{2+}& -1925\\ {\text{Mn}}^{2+}& -1862\\ {\text{Fe}}^{2+}& -1998\\ {\text{Co}}^{2+}& -2079\end{array}$